This invention relates to thin film resistors and more particularly to thin film resistors having a low temperature coefficient of resistance (TCR), good stability, and good resistance to corrosion.
Commercially available thin film resistors are mainly fabricated from either nichrome (NiCr) or tantalum nitride (Ta.sub.2 N). Unfortunately, performance limitations have been noted with resistors comprising either of these materials. For example, a particularly significant problem observed with nichrome resistors is that chromium oxidizes when exposed to air. Oxidization of chromium causes the TCR of nichrome to become more negative over time and results in a limited shelf life for resistors fabricated from such material. While tantalum nitride resistors do not oxidize as rapidly as nichrome resistors, they too tend to degrade over time at a rate that is unacceptable for certain precision applications.
The rate of degradation of currently available NiCr or Ta.sub.2 N resistors increases when such resistors are exposed to stressful thermal, humidity, or power loading conditions.
In addition to their stability limitations, currently available NiCr or Ta.sub.2 N resistors are unable to withstand exposure to certain chemicals especially common in chemical industry environments. Exposure to HF or HCL vapors, for example, or acids such as H.sub.2 SO.sub.4, will change drastically the electrical characteristics of resistors fabricated from either material.
In the meantime, the unique physical properties of the material ruthenium oxide have attracted increasing attention from researchers and scientists. Recent studies concerning various applications for the material have shown that ruthenium oxide exhibits excellent stability over time, excellent stability when exposed to thermal stress, excellent stability with respect to changes in humidity, excellent diffusion barrier characteristics, and excellent resistance to corrosion upon exposure to certain chemicals, even against chemicals that other materials traditionally employed as corrosion barriers, such as Ta.sub.2 N, are unable to withstand.
Thick film resistors utilizing ruthenium oxide have been thoroughly investigated and are currently available. However, applications for such resistors are severely limited because of disadvantages inherent in thick film resistor processing. As compared to thin film resistors, it is well known that thick film resistors are unreliable, large, and have poor high frequency response. Furthermore, a variance of resistivity of about 20% is unavoidable with thick film resistors owing to the distribution of thickness of the thick film and unstable firing conditions. These problems inherent with thick film processing make thick film ruthenium oxide resistors unsuitable for many applications including most precision applications.
In light of the limitations of materials presently used in thin film resistor systems, and further in light of the inherent disadvantages of thick film resistor processing, it is a principal object of the present invention to provide a thin film resistor comprising ruthenium oxide which, by way of easily reproduced process controls, exhibits a near-zero temperature coefficient of resistance, and which, in addition, exhibits excellent stability over time (shelf-life stability), excellent stability when exposed to stressful thermal, humidity, or power loading conditions, and excellent resistance to chemical erosion.
Other objects of the present invention are to provide a simplified fabrication process for stable thin film resistors, to provide a resistor with a good termination layer for metallization, and to provide a resistor which can be assembled and packaged flexibly and economically.
Further objects of the present invention will become apparent from the ensuing description.